The duty performed by an aircraft power plant has generally in the past been monitored by maintaining in a log book the number of hours of aircraft operation. Such records have customarily been used to obtain an indication when it is essential to overhaul the power plant. Not only are such prior art practices subject to error due to inaccuracies in the manual record keeping, but also such methods do not give a reliable indication of the extent of work performed by the aircraft power plant. Further, in some conditions of service, such as when operating in a hostile environment it is not practical to maintain accurate records of aircraft operation.
It has thus been proposed to equip an aircraft engine with a temperature-time integrating counter to provide an automatic and more reliable indication of the actual duty performed by the engine. This improved accuracy results from the fact that the rate of consumption of the operating life of a gas turbine engine is proportional to the product of the temperature at which it operates and the duration of that temperature. While such devices provide improved measure of consumed engine operating life there remains room for improvement for such devices. In particular, other criteria exists on which a judgment may be made as to whether an aircraft engine should be removed for overhaul.